This invention relates to heat exchangers, and in particular, to by-pass valves for by-passing a heat exchanger in a heat exchange circuit under conditions where the heat transfer function of the heat exchanger is not required or is only intermittently required.
In certain applications, such as in the automotive industry, heat exchangers are used to cool or heat certain fluids, such as engine oil or transmission fluid or oil. In the case of transmission fluid, for instance, a heat exchanger is used to cool the transmission fluid. The heat exchanger is usually located remote from the transmission and receives hot transmission oil from the transmission through supply tubing, cools it, and delivers it back to the transmission again through return tubing. However, when the transmission is cold, such as at start-up conditions, the transmission oil is very viscous and does not flow easily through the heat exchanger, if at all. In such cases, the transmission can be starved of oil and this may cause damage or at the least erratic performance. Cumulative damage to the transmission can also occur if the quantity of oil returned is adequate, but is overcooled due to low ambient temperatures. In this case, for instance, moisture condensation in the oil (that would otherwise be vaporized at higher temperatures) may accumulate and cause corrosion damage or oil degradation.
In order to overcome the cold flow starvation problem, various solutions have been proposed in the past. One solution is to use a small by-pass conduit acting as a short circuit between the heat exchanger supply line and the return line to the transmission. While this provides for some cold flow and prevents the transmission from being starved of oil, it reduces the heat exchange efficiency when the transmission fluid reaches operating temperature, because some of the transmission fluid does not go through the heat exchanger. The problem is exacerbated by the changing transmission oil viscosity as the oil heats up. In other words, a by-pass channel large enough to give sufficient flow to prevent starvation at cold temperatures produces too much by-pass flow when the oil heats up and becomes less viscous.
In order to overcome the reduced efficiency caused by simple by-pass channels, it has been proposed to put an actual shut-off valve in the by-pass line.
When the oil is cold, the by-pass channel is open, and when the oil heats up, the valve in the by-pass line closes to prevent further by-pass. Usually some type of temperature responsive valve element is used, such as a bimetallic strip or some other type of device that expands and contracts or rotates to open and close the by-pass valve when the oil temperature exceeds certain limits.
There are several difficulties with the by-pass valves used in the past. With some types of valves, when the valve closes and the by-pass flow stops, the element that senses the oil temperature and causes the valve actuation is either taken out of the flow path or overly exposed to the flow path. In the former case, the valve actuator loses its ability to sense accurately the oil temperature and open and close when desired. In the latter case, the valve actuator is exposed to the oil at the extreme ends of the oil operating temperature range, and this can permanently damage some types of actuators.
Another problem with the actuator valves used in the past is that when they open and close, they can cause pressure peaks or spikes in the flow circuit. This is highly undesirable in transmission oil cooling, because transmission operation is sensitive to pressure, and pressure spikes affect the shifting of the transmission.
The present invention overcomes many of the above problems by placing the by-pass valve temperature responsive actuator in a valve chamber and having the supply or return flow to the heat exchanger pass through this chamber. The by-pass flow also passes through the chamber.